1. Field of the Invention
The present invention relates to a touch panel device that can be used as an input device for equipment such as a personal computer. More specifically, the present invention relates to a touch panel device having plural pairs of surface acoustic wave transmission elements and reception elements, each of which includes a comb electrode and a piezoelectric thin film, for detecting a touched position.
2. Description of the Prior Art
Such a touch panel device can provide a user-friendly input interface being combined with a display device such as a CRT or an LCD. For example, a command input or selection can be performed easily by touching a button or an icon displayed on a screen of the display device.
FIG. 9 shows a simplified structure of a conventional touch panel device utilizing a surface acoustic wave (also referred to as “SAW”). On the middle portion of a substrate 51 such as a glass plate, a rectangular area 52 is provided as a touch area, around which plural pairs (ten pairs in FIG. 9) of comb electrodes 53 and 54 are arranged. The comb electrodes 53 that made up the SAW transmission elements are disposed along the upper side and the left side of the rectangular area 52, while the comb electrodes 54 that made up the SAW reception elements are disposed along the lower side and the right side of the rectangular area 52.
A piezoelectric thin film 55 is formed in the area of the comb electrodes 53 and 54 around the rectangular area 52. The piezoelectric thin film 55, which is a zinc oxide thin film, for example, is formed so as to cover the substrate 51 and the comb electrodes 53 and 54 formed on the substrate 51. Alternatively, the piezoelectric thin film 55 is formed on the substrate 51, and the comb electrodes 53 and 54 are formed on the piezoelectric thin film 55.
When predetermined amplitude of excitation voltage is applied across the electrodes of the transmission comb electrode 53, a surface acoustic wave signal is generated. This surface acoustic wave signal propagates on the piezoelectric thin film 55 and the substrate 51 toward the corresponding reception element including the comb electrode 54 and the piezoelectric thin film 55, as shown by a line with an arrow. Then, a reception voltage signal is outputted from the comb electrode 54 of the reception element.
If a finger touches a certain position on the surface of the substrate 51 in the rectangular area 52 where surface acoustic wave signals propagate as shown by the lines with arrows, the propagation of the surface acoustic wave signal is intercepted. As a result, the amplitude of the reception voltage signal obtained from the comb electrode 54 of the reception element is attenuated substantially. Thus, in the conventional device shown in FIG. 9, a touched state or a non-touched state (i.e., a touched position) on the surface of the substrate 51 can be detected for 25 positions, which are intersections of five vertical propagation paths and five horizontal propagation paths as shown by lines with arrows.
FIG. 10 shows a simplified structure of another conventional touch panel device, in which a pair of the transmission comb electrode 53 and the reception comb electrode 54 are placed on the neighboring sides of the rectangular area 52 instead of the opposing sides thereof. Therefore, the surface acoustic wave signals propagate from the transmission comb electrode 53 to the reception comb electrode 54 in a slanting direction (in the diagonal direction of the rectangular area 52). For this reason, the comb electrodes 53 and 54 facing each other are arranged in a slanting direction with respect to four sides of the rectangular area 52.
In this way, the distance between the neighboring comb electrodes can be shortened for increasing a resolution of the touch panel compared with the structure shown in FIG. 9, using the same size of the comb electrodes 53 and 54. In addition, since the comb electrode pairs 53 and 54 have different propagation path lengths (i.e., different propagation times), the reception comb electrodes 54 receive the surface acoustic wave signal at different timings even if the plural transmission comb electrodes 53 are excited simultaneously. Thus, the difference of the propagation time among the propagation paths can be utilized for detecting a touched position.
However, the structure shown in FIG. 10 has a disadvantage. A contour of the piezoelectric thin film 55 formed in the area of the comb electrodes 53 and 54 around the rectangular area 52 becomes complicated. With the structure shown in FIG. 9, the inside edge of the piezoelectric thin film 55 can be linear along the four sides of the rectangular area 52. However, in the structure shown in FIG. 10, the inside edge of the piezoelectric thin film 55 should be shaped zigzag (step-like) along the contour of the comb electrodes 53 and 54 that are placed in a slanting direction with respect to the four sides of the rectangular area 52.
In any case, the edge of the piezoelectric thin film 55 must be perpendicular to the propagation path between the pair of comb electrodes 53 and 54. Otherwise, a refraction is generated due to the difference of a propagation speed of the surface acoustic wave at the boundary between the area with the piezoelectric thin film 55 and the area without the piezoelectric thin film 55 (i.e., the area of the substrate 51), so that the surface acoustic wave signal cannot propagate efficiently from the transmission comb electrode 53 to the opposite reception comb electrode 54.
In order to improve the resolution of the touch panel, a distance between the neighboring propagation paths (i.e., an arrangement pitch of the comb electrodes) should be shortened. Then, the zigzag shape of the edge of the piezoelectric thin film 55 becomes finer, and higher accuracy in the process is required. Higher accuracy is also required in registration of the zigzag shape of the edge of the piezoelectric thin film 55 with the positions of the comb electrodes 53 and 54. As a result, yield may drop and manufacturing cost may increase.